1. Field of the Invention
This invention relates to silver halide photographic materials and, more particularly, it relates to photographic materials providing very high contrast and negative image photographic characteristics.
2. Description of the Prior Art
It is known as disclosed in, for example, U.S. Pat. No. 2,419,975 that high contrast and negative image photographic characteristics can be obtained by adding a hydrazine compound to a silver halide photographic emulsion. U.S. Pat. No. 2,419,975 discloses that very high contrast photographic characteristics of a gamma over 10 are obtained by adding a hydrazine compound to a silver chlorobromide emulsion and developing the photographic emulsion with a developer having a pH as high as 12.8. However, a strongly alkaline developer having a pH near 13 tends to be aerially oxidized and unstable, and hence cannot be stored and used for a long period of time. Also, development at such a high pH tends to cause fog.
U.S. Pat. No. 3,386,831 describes a process for stabilizing an emulsion by adding a mono-phenylhydrazide of an aliphatic carboxylic acid into an essentially surface-sensitive photographic silver halide emulsion. The object of U.S. Pat. No. 3,386,831 is to stabilize the emulsion and it differs from the objects of the present invention.
Super-high contrast photographic characteristics are very useful for the photographic reproduction of continuous tone images by dot images, which are useful for printing plate making, or the reproduction of line images in both cases of a negative image and positive image. For the purpose, a process has generally been employed in which a silver chlorobromide photographic emulsion having a silver chloride content of more than about 50 mole %, preferably more than 75 mole %, is used and the photographic emulsion layer is developed with a hydroquinone developer having a very low effective concentration of sulfite ion (usually lower than 0.1 mole/liter). However, since the sulfite ion concentration of the developer is low in the process, the developer is very unstable and cannot be stored more than 3 days. Furthermore, since a silver chlorobromide photographic emulsion having a comparatively high content of silver chloride must be used in the above-described process, it is difficult to obtain high sensitivity.
Therefore, providing super-high contrast photographic characteristics useful for the reproduction of dot images and line images using a highly-sensitive silver halide emulsion and a stable developer has been strongly desired.
It has already been found that by incorporating a compound represented by the general formula (I) EQU R.sup.1 NHNHCOR.sup.2 (I)
wherein R.sup.1 represents an aryl group and R.sup.2 represents a hydrogen atom, a phenyl group, or an unsubstituted alkyl group having 1 to 3 carbon atoms; in a negative image silver halide photographic emulsion containing substantially surface latent image type mono-dispersed silver halide grains having a mean grain size of not over 0.7 micron, super-high contrast negative photographic characteristics of a gamma as high as sometimes over 10 could be obtained using a stable developer having a comparatively high concentration of sulfite and a pH which is not very high e.g., as described in our U.S. Patent Applications Ser. Nos. 804,484, filed June 7, 1977 and Ser. No. 823,881, filed Aug. 11, 1977, (corresponding to Japanese Patent Application Nos. 66,354/76 and 96,337/76.) However, since the silver halide grains of the high contrast photographic emulsion are limited to those having a mean grain size of not over 0.7 micron, the sensitivity of the silver halide photographic emulsion obtained is not sufficiently high even due to the sensitization action by this compound represented by the general formula (I).
Japanese Patent Application (OPI) Nos. 63,914/'75; 36,130/'76 and 77,223/'76 disclose that some tetraazaindene compounds having a hydroxyl group increase the sensitivity of silver halide photographic emulsions. These compounds have been long well known as stabilizers for preventing the properties of silver halide photographic materials from changing during storage.